This invention relates generally sonar systems and more particularly to sonar systems adapted to map the bottom of a body of water and to identify submerged foreign objects.
As is known in the art, it is sometimes desirable to identify submerged foreign objects such as mines, cables or oil pipe lines using sonar. A common sonar type used for examining the sea bottom is a Side Looking Sonar (SLS). A SLS is either towed or mounted on an underwater vehicle and is moved through the water in a forward direction at an approximately constant speed and depth. The sonar transmits a short (typically 0.10 to 0.20 ms), high frequency (typically 500 kHz) pulse into the water and has a very narrow horizontal beam-width (typically 1 degree or less) in a direction perpendicular to the forward direction. The pulse propagates through the water, reflects off of the sea bottom and the echo returns to the sonar. After transmission, the sonar begins receiving the echoes. Echoes that arrive later in time come from further away on the sea bottom. The received signal maps to a long, thin strip of the sea bottom and is called a range scan. After a fixed elapsed time, and after the vehicle has moved a short distance in the forward (or cross-range) direction, the sonar stops receiving and begins a new transmission. The length of the fixed elapsed receive time determines the maximum range of the sonar along the sea bottom. The range may be also limited by the sonar power. Because of spreading and absorption loss, the received intensity decreases with range (time elapsed from transmission). This is compensated for in the sonar by a Time Varying range-variable Gain (TVG). The beamwidth and pulse length determine the sonar's azimuth and range resolutions, respectively. As the sonar moves in the forward (i.e. cross-range) direction, the range scans correspond to subsequent parallel strips along the sea bottom thereby producing a two dimensional "map" of the sea bottom: sonar received intensity (i.e. the z axis) vs. range (i.e. the x-axis) and cross-range (i.e. the y-axis). A SLS sometimes transmits and receives on both the port and starboard sides and produces two images.
Because the sonar travels at a certain altitude above the sea bottom, the first echoes are very faint and are a product of volume scattering in the water between the sonar and the sea bottom. This is called the water column and its length (in time) depends on the sonar's altitude. These faint echoes give no information about the sea bottom and are therefore removed from the scan data. What remains is the two-dimensional "map" of the sea bottom: sonar received intensity vs. range and cross-range; this is called the raw image data. Because the grazing angle decreases with range and a shallow grazing angle produces less backscatter, the image intensity decreases with range. This is apparent in the raw image data: the near range data is much more intense than the far range data. The raw image data is normalized to eliminate the effect of grazing angle to form the normalized image data.
The normalized image data can be thought of as a map of the sea bottom but this can be misleading. Although elevations will often produce stronger echoes (and therefore higher image intensity) and depressions will often produce weaker echoes (lower image intensity), echo intensity is also affected by the reflectivity of the sea bottom, the texture of the sea bottom and the local grazing angle at the sea bottom.
A mine on the sea bottom may produce a region of high intensity in the image (highlight) by reflecting directly back to the sonar. It may also produce a region of low intensity in the image (shadow) by blocking the sea bottom beyond itself from ensonification; these shadows are sometimes very long. If a mine is partially buried, it may not reflect any energy back to the sonar but instead reflect it away, this produces a shadow without a highlight. A method is required to analyze these patterns of shadow, background and highlight regions of SLS imagery to recognize the existence of candidate mine objects. Subsequent use of a neutralization system to remove candidate objects which pose obstructions to safe navigation follows the mine recognition processing effort.
Today, mine recognition in sonar imagery is generally performed manually. Human operators are trained to evaluate the high-resolution imagery and look for clues to a mine, which has a set of typical characteristics. Human interpretation is the current state of the art for SLS imaging sonars. The operator must spend considerable time analyzing the data to determine which returns are from mines.